The present invention is an improved apparatus and method for continuous fabrication of single- and multi-wall carbon nanotubes (CNT) as well as other micro- and nano-wires, tubes or filaments by chemical vapor deposition, vapor-liquid-solid, or other similar seeded growth mechanisms as known by one of ordinary skill in the art. Particularly, the present invention is concerned with an apparatus for a segregated chemical vapor deposition (CVD) process where a carbon based gas is decomposed at high temperature in the presence of catalyst whereby a CNT is formed on the catalyst surface.
The first images of nanoscale, tubular carbon filaments were produced in the early 1950's by Russian scientists Radushkevich and Lykyanovich. The tremendous potential impact of carbon nanotubes (CNTs) was not felt until the work of Iijima et. al. [1: Iijima and Ichihashi; Single-shell carbon nanotubes of 1-nm diameter. Nature, 1993, (363), 603-605.] and Bethune et. al. [2: Bethune, Klang, De Vries, Gorman, Savoy, Vazquez and Beyers; Cobalt-catalysed growth of carbon naontubes with single-atomic-layer walls. Nature, 1993, (363), 605-607.] in 1993. Due to the projected impressive electronic and thermal transfer characteristics, CNTs have been of great interest in the electronics community for their potential to improve the functionality of electronic devices. [3: Javey, Guo, Wang, Lundstrom and Dai; Ballistic carbon nanotube field-effect transistors. Nature, 2003 , (424), 654-657.]
The CVD process is often used as a synthesis method for high purity CNTs because of the controllability over precursor flow rate, composition, impurities and temperature. In the CVD process, a hydrocarbon gas such as, but not limited to, ethylene or acetylene is flowed into a reactor with hydrogen gas and a carrier such as, but not limited to, argon. The reactor is heated to a fixed set-point over the decomposition temperature of the selected hydrocarbon gas, at which point the gases decompose and reform into CNTs when in the presence of a catalyst or structure directing matrix. The exact formation mechanism of the CNTs is still of wide debate, and, as discussed later, the our novel apparatus may further aid in understanding of this mechanism although such an understanding is not a prerequisite to implementing the present invention. The ability to grow dense, ordered CNTs is directly related to the ability to evenly distribute the catalyst nanoparticles (NPs) on a substrate. Traditional nanoparticle synthesis methods include sol-gel formation, co-reduction of precursors, thermal decomposition, and physical deposition. These methods can produce large quantities of catalyst NPs, [4: Bacsa, Laurent, Peigney, Bacsa, Vaugien and Rousset; High specific surface area carbon nanotubes from catalytic chemical vapor deposition process. Chemical Physics Letters, 2000, (323), 566-571.], but it can be difficult to obtain the dense, highly ordered arrays of CNTs that are desired for the production of microelectronic devices. The standard method for carpeted growth of CNTs by CVD is to form a thin (˜1 nm) catalyst film, which is difficult to produce over large substrate areas, on a substrate that is designed to interact in a phobic manner with the catalyst upon melting such that molten NPs are formed and CNTs growth from them.
Template-directed growth of catalyst nanostructures and CNTs in anodized aluminum oxide (AAO) porous films can produce dense and highly ordered CNT arrays. To form a templated CNT array, pure aluminum is first anodized in a chilled acid bath via a potentiostatic anodization process. The parameters of the anodization process are tunable to produce AAO pores with diameters of 10-100's nm, lengths up to 3 mm, and pore densities approaching 1012 cm−1. AAO can be formed atop native aluminum or other conductive supports. The growth of CNTs in AAO nanopores is typically performed via a catalyzed CVD process with transition metal catalysts such as cobalt, nickel or iron.
Template assisted CNT growth is commonly performed with catalytic nanoparticles of iron-group elements (Fe, Co, or Ni) or alloys of these with Mo or Pd. Significant attention has been paid to studying the relationship between these catalysts and the CNT structure, quality, and dimensions. To synthesize CNTs in the AAO template, the anodized alumina barrier layer at the pore base was removed so as to expose the conductive aluminum surface for catalyst nanoparticle electrochemical deposition (ECD). After ECD, the template was subjected to CVD at the same conditions described previously. Growth continuing beyond the surface of the template is, however, rapidly disordered and quenched due to catalyst aggregation, poisoning and build-up of carbonaceous material on the surface.
When fashioning devices such as wires and composites from CNTs, ultimate length is the driving factor behind many of the device properties. Traditional growth methods previously described have limited CNT length which, in turn, inhibits the material's potential properties due to the large number of CNT-CNT interfaces acting to increase resistance. By extending the length of the CNT beyond the micron scale up to the centimeter or meter length, the material properties of CNT based wires and composites would near that of pure CNTs.
The many methods for CNT fabrication are limited in ultimate CNT length as CNT growth is stunted by a number of naturally occurring processes. The catalyst material can be poisoned through contaminants in the gas stream or can become encapsulated in carbon residue from the degradation of the hydrocarbon gas stream. Alternately, the CNTs can become entangled among themselves causing catalyst merging and/or premature termination.
U.S. Pat. No. 7,431,965 discloses an apparatus for continuous production of single wall CNTs (SWCNTs) using a metallic catalyst supported on an AAO nanoporous membrane, with pores open on both sides, where the membrane is used to separate the inner tube of a concentric tube reactor and a pressure differential between the tubes is the driving force to the direction of CNT growth. The disclosed mechanism of CNT formation does not, however, appear to take into account the immense capillary pressure (Pc) in a nanopore which is ˜107 Pa according to the equation
      P    c    =            2      ⁢                          ⁢      γ      ⁢                          ⁢      cos      ⁢                          ⁢      θ        r  where γ is the surface tension of the catalyst, θ is the catalyst contact angle, and r is the nanopore diameter. At these pressures, the vacuum pulled on the inner tube 14 does little to influence CNT formation in that direction. Also, the disclosed apparatus does not allow for varying gas flow conditions in that inner tube in order to study the formation and doping mechanisms of CNTs. That apparatus uses a non-ideal flow pattern which provides inconsistent gas flow to the catalyst layer 30.
The present invention is an apparatus for a chemical vapor deposition process for the formation of carbon nanotubes by catalytic decomposition of carbonaceous precursors in the presence of hydrogen at high temperatures. The CNTs are formed through an AAO porous membrane with a thin, solid catalyst layer on one side used to segregate the carbonaceous gases from a clean stream in order to prevent premature termination due to solid carbon accumulation, catalyst poisoning and other termination mechanisms. Since the flow is segregated and variable, the role of hydrogen during CNT growth into the “clean” stream can be studied and elucidate the formation mechanism for CNT CVD growth. Indeed, this apparatus can be beneficially used to enact any kind of catalyst-seeded growth with segregated flow of identical or similar compositions.
An object of our invention is to provide a single heated chamber segregated into two chambers along its length by a plate which may be fixed or removeable. Importantly, the plate has a void such that the porous membrane may be affixed to it in order to provide a single point where gas can diffuse between the chambers through the AAO supported catalyst film. The gas composition of each chamber can be varied to consider various formation and doping mechanisms in CNTs or other nanostructure growth mechanisms. As a result, no pressure drop is required to drive CNT growth into the second chamber as the capillary pressure induced from the interfacial interaction of the molten catalyst and the nanoporous template will be tuned by chemically and/or structurally to accomplish this.
A further object of our invention is to control the catalyst architecture within the nanopores of the template. A thin film of catalyst is deposited on one side of the porous membrane such that gases must diffuse through the catalyst to move between the segregated chambers. The catalyst material forms a meniscus about the nanopores where the shape of the meniscus is related to the interfacial energies between the catalyst and the porous membrane. Controlling this interfacial energy will determine whether the meniscus forms within the pore or at the pore opening and will determine the shape of the meniscus depending on if the porous membrane is phillic or phobic to the catalyst material. The interfacial energy can be tuned by altering the temperature, pore size, or membrane surface chemistry among other techniques known to the art. The catalyst film atop the nanoporous template may be placed in the reactant or clean side of the plate segregating the chamber. The catalyst may also be applied to the porous membrane in other forms such as, e.g., a pore wall coating.
The present invention is a substantial improvement over the known CVD processes for growth of CNTs in such a way to induce ultra-long CNT growth while examining the CNT growth mechanism and doping mechanism in various carbon free gases. Other objects, advantages and novel features of the present invention will become apparent from the following detailed description when considered in conjunction with the accompanying drawings herein.